Geologica Acta: an international earth science journal ISSN: 1695-6133 [email protected] Universitat de Barcelona España

Guimerà, J.; Arboleya, M.L.; Teixell, A. Structural control on present-day topography of a basement massif: the Central and Eastern Anti-Atlas () Geologica Acta: an international earth science journal, vol. 9, núm. 1, marzo, 2011, pp. 55-65 Universitat de Barcelona Barcelona, España

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Structural control on present-day topography of a basement massif: the Central and Eastern Anti-Atlas (Morocco)

1 2 2 J. Guimerà * M.L. Arboleya A. Teixell

1 Departament de Geodinàmica i Geofísica, Facultat de Geologia, Universitat de Barcelona (UB) Martí i Franqués s.n. 08028 Barcelona, Spain. E-mail: [email protected]

2 Departament de Geologia, Facultat de Ciències, Universitat Autònoma de Barcelona 08193 Bellaterra, Spain

* Corresponding author

ABSTRACT

The Anti-Atlas basement massif extends South of the , and, despite a very mild Cenozoic deformation, its altitude exceeds 1500m in large areas, reaching 3305m in Jbel Sirwa. Structural contours of the present elevation of a polygenic planation surface (the High Erosional surface) and of the base of Cretaceous and Neogene inliers have been performed to characterize the major tectonic structures. Gentle Cenozoic WSW-ENE- and N-S- trending folds, of 60 to100km wavelength, reactivate Variscan structures, being the major contributors to the local topography of the Anti-Atlas. Reactivated thrusts of decakilometric to kilometric-scale and E-W trend involving the Neogene rocks exhibit a steep attitude and a small displacement, but they also produce a marked topographic expression. The resulting Cenozoic horizontal shortening along N-S sections across the Anti-Atlas is about 1%. The position of the major anticlinal hinges determines the location of the fluvial divides of the Warzazat basin and the Anti-Atlas, and a structural depression on one of these hinges ( anticline) allowed the formerly endorheic Warzazat basin to drain southwards. The first Cenozoic structures generating local topography are of pre-mid Miocene age (postdated by 6.7Ma volcanic rocks at the Jbel Saghro), whereas the youngest thrust movements postdate the Pliocene sedimentary and volcanic rocks (involving 2.1Ma volcanic rocks at Jbel Sirwa). In addition to these features, the mean elevation of the Anti-Atlas at the regional scale is also the result of a mantle thermal anomaly reported in previous works for the entire Atlas system.

KEYWORDS Cenozoic contraction. Crustal folding. Uplift. Tectonic topography. Anti-Atlas. Morocco.

INTRODUCTION 3305m in the Jbel Sirwa, 2701m in the Amalou-n-Mançour (Jbel Saghro), and 2526m in the Imgand (Central Anti- The Anti-Atlas of Morocco is a mountainous region Atlas). Being dominated by Proterozoic and Palaeozoic located south of the Warzazat and Sous basins, which are the rocks, the Anti-Atlas contains occurrences of marine southern foreland basins of the High Atlas (Fig. 1). The present Cretaceous sediments indicating that such elevations are topographic elevation of the Anti-Atlas is over 1500m in the signal of a Cenozoic uplift. However, much of the extensive areas, exceeding 2000m in several places (Fig. 2): internal structure of the Anti-Atlas was formed during the

55 J. GUIMERÀ et al. Topography of a basement massif: the Anti-Atlas

6º W W

N MEDITERRANEAN SEA A

O C E N

C

I

T

N

A

L

T

A High Atlas i-Atlas ATLAS Ant Hama 8º W

Tinejdad da HIGH du Guir sin ch arzazat Ba rake W Mar Warzazat S L A T A Zagora Taliwine in I - T em Sous Bas N Thrust Fig. 4 A Kem-K South Atlas Thrust Fig. 8 Cenozoic Cretaceous Tata 0 50 100 km Triassic and Jurassic Fig. 2 Pre-Mesozoic rocks

FIGURE 1 Geological map of the Anti-Atlas and the Central and Western High Atlas, simplified after Hollard (1985). Location of Figs. 2, 5 and 8 is shown. Fig 1

Panafrican and Variscan orogenies (late Proterozoic and late this study illustrates how a geomorphological analysis can Palaeozoic, respectively) while the Cenozoic deformation provide grounds to the tectonic interpretation in regions is considered to be very mild (Choubert & Marçais, 1952; where the sedimentary record is scarce. Hoepffner et al., 2005).

The high topography of the High Atlas thrust belt is GEOLOGICAL SETTING the combination of crustal shortening and long-wavelength thinning of the lithosphere, affecting the mountain belt Geologically, the Anti-Atlas is composed of a and the peripheral plains on a much wider scale (Teixell Proterozoic Pan-African basement overlain by post-Pan- et al., 2003, 2005; Missenard et al., 2006). The differential African Uppermost Proterozoic and Palaeozoic rocks, topography between the High Atlas and the adjacent deformed during the Variscan orogeny (Choubert, 1952; elevated plains results from tectonic shortening, manifested Choubert & Marçais, 1952). The Variscan deformation by well developed imbricate thrust systems. This is not so caused in the inversion of late Proterozoic and Palaeozoic straightforward in the Anti-Atlas, where these systems are intracratonic basins, remelting in a thick-skinned fold lacking. Choubert (1952) described the overall structure of and-thrust-belt (Burkhard et al., 2006; Toto et al., 2008; the Anti-Atlas as a vast basement arch (pli de fond), which Guimerà & Arboleya, 2008). resulted from the remobilization of a previous fold of Variscan age, a view followed by Teixell et al. (2003), who Directly overlying Precambrian and Palaeozoic rocks, a interpreted it as a lithospheric-scale fold. A closer inspection Cretaceous succession starts with pre-Cenomanian fluvial evidences that topography is not homogeneous within the red beds, followed by Cenomanian-Turonian marine Anti-Atlas (Fig. 2) and, as pointed out by Choubert (1952), limestones and red-beds of Senonian age (Choubert, 1952). major faults do involve Mesozoic and Cenozoic rocks. The pre-Cenomanian red beds, markedly unconformable The aim of this paper is to analyze the tectonic origin of on the pre-Mesozoic basement, cover a peneplain (Robert- topography on the basis of 1) the attitude of several surface Charrue, 2006) and are preserved in isolated outcrops, markers that can be reconstructed from preserved erosional essentially on the northern and eastern side of the Anti- features and sedimentary inliers within the Anti-Atlas, and Atlas (Choubert, 1950; Choubert, 1955-56, Ferrandini et 2) the field analysis of the contractional structures that al., 1985) (Fig. 1). These pre-Cenomanian clastic sediments involve Neogene rocks and morphological surfaces. In any derive from the region, and were deposited by case, Post-paleozoic rocks are rare in the Anti-Atlas, and alluvial systems, whose regional slope was north-directed

Geologica Acta, 9(1), 55-65 (2011) 56 DOI: 10.1344/105.000001643 J. GUIMERÀ et al. Topography of a basement massif: the Anti-Atlas

(Guillocheau et al., 2007). The Cenozoic is largely confined dips steeply to the North and had a reverse slip during the to the Warzazat and Sous basins (Fig. 1), where it begins Neogene (Missenard, 2006; Sébrier et al., 2006; Guimerà with Palaeocene to Middle Eocene marine limestones, et al., 2006 and 2008) (Fig. 3 B; Fig. 5, section 2). followed by continental Eocene red beds (Choubert, 1950; Choubert, 1955-56; Gauthier, 1957). After a marked hiatus, the Aït Kandoula Formation (El Harfi et al., 2001) METHOD OF STRUCTURAL ANALYSIS is composed by alluvial and lacustrine terrigenous and carbonate sediments of Middle to Upper Miocene age In view of the scarcity of post-Palaeozoic deformation (Tesón, 2009). The Aït Kandoula Fm. constitutes the bulk markers, the method of analysis of the structures of the infill of the Warzazat basin (ca. 1000m), and crops responsible for the uplift of the Anti-Atlas consists of out in isolated exposures in the Sirwa massif, the western structural contour mapping of reference surfaces. More part of the Sous basin, and within the Anti-Atlas (Choubert, than 900 points of elevation measurements were acquired 1945; Gauthier, 1957; Görler et al., 1988), where it from SRTM90 DEM (about 90m of pixel size, available constitutes a valuable marker for Neogene deformation. It at http://srtm.csi.cgiar.org) and our own field-based has not been documented whether the Anti-Atlas was ever geological mapping on satellite orthoimages. After this, we completely covered by Mesozoic or Palaeogene sediments. have drawn georeferenced contour maps of the present-day Previous authors have assumed that most of it was exposed elevation of the sub-Cretaceous unconformity, the HES during these epochs (Choubert, 1952; Riser, 1988; Robert- and the base of the Neogene, using Surfer software with Charrue, 2006; Malusà et al., 2007). the Kriging gridding method. The results of contouring are presented in Figs. 4, 6 and 8. A characteristic feature of the Anti-Atlas is a widespread planation surface called the High Erosional Surface (HES) For the sub-Cretaceous unconformity and the base of or Pre-hamadian Surface by Choubert, (1952). This the Neogene, elevation measurements were acquired from author recognized this surface as polygenic in origin and geological maps. For the HES, elevation measurements attributed its development to an indeterminate period from were taken, after the recognition of this surface in the field the beginning of the Tertiary to the Aquitanian. Moreover, (Fig. 3 A, B, C and F), from 3D visualizations of satellite we have observed that North of the Sirwa region the orthoimages draped on the SRTM90 DEM. HES derives from the exhumation of the pre-Cretaceous peneplain (Fig. 3 A). What can be ascertained is that this Taking into account that the Aït Kandoula Fm. and HES predates the sedimentation of the Aït Kandoula Fm. the Neogene volcanic rocks are found both on top of (mid-Miocene), which is either deposited on top of the the HES and filling paleovalleys downcut into it, the HES (Sirwa massif and Draa valley; Fig. 3 A, B and C) or present differences in elevation of their base is the confined within paleovalleys downcut into it (northern part result of both the Neogene uplift and the filling of of the Anti-Atlas; Choubert, 1952; Görler et al., 1988; Fig. a paleorelief. This must be taken into account in the 3 C and D). Regardless of its origin, we have mapped this structural interpretation. remarkably flat surface -which Choubert (1952) defined as a peneplain- as a single entity. This generalized planation We have constructed generalized geological cross- surface is well preserved in large areas of the Anti-Atlas, sections to illustrate the attitude of the contoured markers over Precambrian and Palaeozoic rocks (Fig. 3 A, B, C and to define the Cenozoic deformation structure (Fig. 5). and F), its presence implying a low relief region before the recent uplift of the Anti-Atlas.

Late Miocene to Pliocene phonolitic and trachytic N volcanism (11 to 2Ma; Berrahma and Delaloye, 1989) Hamada du Guir High Atlas as Thrust occurred extensively in the Jbel Sirwa, where the remains Atl South of a major volcano reach 3305m. Miocene to Pliocene (10 2701 asin azat B Saghro Warz SA K to 2.8Ma) nephelinites, phonolites and alkaline basalts are K Sirwa SGA 3305 K us K found in the Jbel Saghro too (Berrahma et al., 1993). So in Bas AAMF Central Kem-Kem K rm 2526 Anti-Atlas K Mesozoic and Neogene rocks around and inside the aran platfo Anti-Atlas Sah 0 100 km Anti-Atlas are cut by faults of decakilometric- to kilometric- CAM scale and dominant E-W strike (Fig. 3 B and E, Fig. 4). The FIGURE 2 STRM90 digital elevation model of the area studied. The main fault is the Anti-Atlas Major Fault, inherited from the mainFig geographic 2 and tectonic features are shown. AAMF: Anti-Atlas Panafrican orogeny, which vertically offsets the Neogene Major Fault; CAM: Central Anti-Atlas Monocline; SA: Sirwa Anticline; rocks some 500m (Choubert & Marçais, 1952). This fault SGA: Saghro Anticline. For location, see Fig. 1.

Geologica Acta, 9(1), 55-65 (2011) 57 DOI: 10.1344/105.000001643 J. GUIMERÀ et al. Topography of a basement massif: the Anti-Atlas

S N N + V HES K HA

Ô Ô

A

NW SE N HES V AMF

Ô V Ad Ad B

SW NE HES N + V

Ô

C N + V

NW SE E K N D

N S W E HES Ô Ô N Ad

E F FIGURE 3 Field images of the area studied. pe: Precambrian. Ad: Adoudounian (latest Proterozoic). K: Cretaceous sediments. E: Eocene. N: Neogene sediments. V: Neogene volcanic rocks. HES: High Erosional Surface. AMF: Anti-Atlas Major Fault. HA: High Atlas. For location, see Fig. 4. A) View of the High Erosional Surface (HES) in the northeast Sirwa area, where it derives from the exhumation of a pre-Cretaceous peneplain. The erosion of the Creta- ceous rocks uncovered a very flat erosional surface, which became a part of the HES. To the West, this surface is covered by the Neogene and volcanic rocks of the Sirwa (see Fig. 3 B). B) The Anti-Atlas Major Fault South of the Sirwa region. Neogene sedimentary and volcanic rocks have a vertical offset of 500m related to the reverse slip of the fault. In the uplifted hanging wall, the High Erosional Surface is preserved beneath Neogene rocks. C) East limb of the Sirwa NNW-SSE anticline. In the foreground, interbedded Neogene sedimentary and volcanic rocks fill a paleovalley downcut in the Precambrian rocks (in the middleground). On top of these Precambrian rocks, the HES is preserved and, over it, in the background, Neogene sedimentary and volcanic rocks, younger than the previous ones, lie on the HES. D) Western end of the Warzazat basin. Horizontal Neogene sedimentary rocks (Aït Kandoula Fm.) are entrenched in folded Eocene and Upper Cretaceous rocks. The town is Tizgzaouine. The High Atlas appears in the background. E) South boundary of the Hmam basin, inside the Anti-Atlas. Neogene sedimentary rocks are tilted 5º 9 to the S and thrusted by Precambrian rocks. F) The HES developed on top of Precambrian rocks around the area where the Draa river crosses the Anti-Atlas relief. The Draa, flowing south, is downcut about 200m.

Geologica Acta, 9(1), 55-65 (2011) 58 DOI: 10.1344/105.000001643 J. GUIMERÀ et al. Topography of a basement massif: the Anti-Atlas

CENOZOIC STRUCTURE AND RELIEF (Fig. 4). Therefore, both limbs are well defined and the fold hinge is located around the ridge crest of the Jbel Saghro. The combination of field observations and the analysis In addition, two isolated buttes (called N and S Tourt) of of the contour maps have enabled us to describe the major Neogene lacustrine limestones and marls are preserved Cenozoic structural elements of the Central and Eastern Anti- almost 30km south of the fold hinge, at 1100m in altitude Atlas. In addition to previously reported features, such as the (easternmost side of Fig. 4) (Choubert, 1959); according Anti-Atlas Major Fault, we have identified a set of large-scale to Riser (1988), these rocks are Miocene in age. Near the folds, huge tilted blocks, and major geomorphic features. base of the South butte, we have found a thin level (0.5m) of interbedded, very altered volcanic rocks. The Saghro Main folds anticline is 200 km-long and has a 70km-wavelength. Its gentle attitude (4º dip of its southern limb: 800m in The Saghro anticline 11.4km), coinciding with the tighter Variscan Sagrho anticline of Choubert (1952) and representing, therefore, South of the Warzazat basin, the contoured surfaces a reactivation of the late Palaeozoic structure. define a large-scale WSW-ENE-trending major anticline which coincides with the relief of the Jbel Saghro The hinge of the Cenozoic fold of the Saghro anticline (Figs. 4 and 5A). The northern limb of the anticline is plunges to the ENE, as deduced from the slope of the HES defined by the NNW-directed slope of the HES and in the eastern termination of the Jbel Saghro (Figs. 4 and the base of Neogene North of the Jbel Saghro (Fig. 4), 5 C). The continuity of the Palaeozoic succession in the whereas the southern limb is evidenced by the S-directed southern limb of the Saghro anticline, well established slope of the HES (2400 to 1600m) to the SE of the Jbel Saghro in geological maps (Choubert, 1959; Faure-Muret and

Erosional�escarpment Contours�(in�meters)�above�sea-level�of: 5A High�Erosional�Surface�of�the Anti-Atlas Neogene�sedimentary�(N)�and�volcanic�rocks Cenozoic�structures: Cretaceous�and�Palaeogene Thrust�and�fault Triasic�and�Jurassic K Anticline Palaeozoic�rocks�in�the Anti-Atlas J Syncline Proterozoic�rocks�in�the Anti-Atlas Monocline

Atlas�South�Thrust

5B Fluvial�network�divide

N HIGH ATLAS K

7B K 1900

K 5C 2400K

K 1700 K 1700 Warzazat�basin Sirwa�Anticline K

K 1300

2200 3A 3D 1600 K 1900 A K N

K 3E A 1400 1800 (1100 m) 2300 3C 1500 Saghro Anticline

1900 K K 1600 Sous�basin K 3F

1700 5C

3B 1800 1850 Anti-Atlas�Major�Fault Draa River 1900

2000 K K Central�Anti-Atlas�Monocline 0 100�km 7A

FIGURE 4 Map displaying the major Cenozoic structures of the study region. Also, the contours of the High Erosional Surface are displayed. Geological contacts are taken from Hollard (1985). N: Neogene. The location of pictures of Figs. 3 and 7 and of sections of Fig. 5 is shown. For location, see Fig. 1.

Geologica Acta, 9(1), 55-65 (2011) 59 DOI: 10.1344/105.000001643 J. GUIMERÀ et al. Topography of a basement massif: the Anti-Atlas

Choubert, 1974-77), indicates that no major thrusts are rocks preserved is the result of the recent erosion by the deeply exposed at the surface (Fig. 5 C). incised (reaching 1000m of incision) Assif-n-Tfnout valley.

The Sirwa anticline Cretaceous rocks north of the Sirwa massif are also folded with a similar pattern (Fig. 6). At a smaller scale, In the Sirwa massif, the contours of all the reference these rocks are also affected by hectometric- to kilometric- surfaces define a broad NNW-SSE-oriented anticline, some scale thrusts and folds oriented N-S and E-W (Fig. 6). 100km in wavelength, which coincides with the topographic high between the present-day Warzazat and Sous basins Central Anti-Atlas monocline (Figs. 4 and 6). The fold amplitude, as defined by the contours of the HES (covered by the Neogene sedimentary and volcanic A gentle WSW-ENE-trending and SSE-vergent rocks), is close to 1000m. In the eastern limb, the HES and the monocline coincides with the south-facing topographic step base of the Neogene rise from 1300m in the western Warzazat of the Central Anti-Atlas (Figs., 4, 5 B and 7 A). As in the basin, to 2300m in the fold hinge (Figs. 4 and 6). In the Jbel Saghro anticline, Palaeozoic succession is continuous western limb, the HES drops from 2300m to 1800m, defining across the monocline, being not disrupted at the surface by a west-directed slope. Moreover, 30km to the west of the last any major thrust (Choubert, 1950-56; Choubert, 1959). Neogene outcrop of the Sirwa massif, one small outcrop of Neogene sedimentary rocks is preserved at an elevation of 725m Main thrusts (Figs. 5C and 6) (Choubert, 1955-56). This outcrop is actually the eastern tip of the Sous basin, and shows lacustrine limestones Thrusts of arguable Cenozoic age are scarce in the dipping 5º-10º to the W. The 30km-wide area with no Neogene Anti-Atlas. The previously described Anti-Atlas Major Fault,

A Erosional Neogene escarpment S Anti-Atlas High Atlas N Cretaceous x10 Jbel Warzazat 3000 Saghro basin Paleozoic 2000

1000 Precambrian 0

B Surface markers of Cenozoic deformation: NNW Base of Neogene (N) SSE Anti-Atlas High Atlas x10 High Erosional Surface 4000 Jbel Sirwa Base of Cretaceous (K) 3000 AMF CAM 2000

1000 100 km 0

C SSW NNE

Souss Basin Sirwa Draa river Saghro x10 Erosional escarpment 3000 N (725 m) K 2000

1000

0

FIGURE 5 Geological cross-sections of the Central and Eastern Anti-Atlas. See Figs. 4 and 8 for location. Above is the section at the real scale, and below, ten times vertically exaggerated sections are shown (dips are distorted according to the exaggeration applied). In section A, the base of the Neogene was taken from Faure-Muret et al. (1994). K: Cretaceous sediments, QB: Quaternary basalts. AMF: Anti-Atlas Major Fault. CAM: Central Anti- Atlas Monocline.

Geologica Acta, 9(1), 55-65 (2011) 60 DOI: 10.1344/105.000001643

Fig 5 J. GUIMERÀ et al. Topography of a basement massif: the Anti-Atlas

a South-vergent thrust bordering the Sirwa massif, appears to 700m-high and over 75km-long with a NE-SW trend clearly expressed by a step of some 500m in the Neogene (Fig. 4, 5 A and 7 B). Its development is previous to the contours (Figs. 4, 5 C and 6). This step is still well expressed volcanic rocks mapped by Faure-Muret & Choubert, in the present topography (Fig. 3 B), as reported by Choubert (1974-77) at its foot, at Ikniwn (2000m), and its top, at the (1952) and Missenard (2006). Other thrusts in the area, smaller Amalou-n-Mançour mountain (2700m) (Fig. 7 B). These in dimensions, are the north-directed Hmam thrust, which volcanic rocks have a late Miocene K-Ar radiometric age bounds to the south the Neogene Hmam basin (Fig. 3 E and of 6.7Ma and 6.6Ma respectively (Berrahma et al., 1993). Fig. 6) and the Qal’at thrust, also north-directed, which cuts the This erosional feature marks the southern outcrop boundary Sirwa volcano (Fig. 6). The trace of these is 15 to 20km long, of the Aït Kandoula Fm. in the Warzazat basin (Figs. 4 and and their vertical slip is of the order of tens of metres. The dip 5 A). The occurrence of coarse grained conglomerates of of the thrust surfaces is 65-70º to the S in both cases. The actual Anti-Atlas rhyolites and andesites in the southern Warzazat dip of the Anti-Atlas Major Fault is difficult to determine, but it basin (Gauthier, 1957; Schmidt, 1992) is consistent with also appears to be very steep. Therefore the shortening associated the existence of a marked relief in the Saghro area and with to all these structures is very limited. Volcanic rocks of the Sirwa the development of the erosional escarpment. massif as young as 2.1Ma (Berrahma and Delaloye, 1989) are affected by the reverse slip of the Anti-Atlas Major Fault. The eastern end of the Anti-Atlas and the Hamada du Guir The Jbel Saghro escarpment Cretaceous rocks north and south of the eastern On the northern slope of the Jbel Saghro, the HES is termination of the Anti-Atlas define a large and very gentle truncated by a large North-facing erosional escarpment up anticline, the limbs of which dip 2º-3º to the N and S.

Contours (in meters) above sea-level of: Base of Neogene sedimentary and volcanic rocks High Erosional Surface of the Anti-Atlas Base of Upper Cretaceous rocks N Neogene sedimentary (Pt) and volcanic rocks 2000 1500

Cretaceous and Palaeogene 2200 Triasic and Jurassic Palaeozoic rocks in the Anti-Atlas K 1900 Proterozoic rocks in the Anti-Atlas 1800 1500K Thrust and fault 2300 1400

Anticline K 1300 Syncline 1800 1200

K 1600

2200 HB 1300

2200 2000 A 1500 1400

2000 K 2300 A

1800 1900 HF 1800 1600

1700 K

2400 1700 1800

1900 2000 y QF 1900 K 1900 2200 (900 m) 2300 1800 1800 1700

1600

N 1700 (725 m) 1900 1600 1700 1800

0 25 km West of this contour, the base of the Neogene coincides with the HES

FIGURE 6 Map of the Sirwa massif and surrounding areas. Contours of the High Erosional Surface, the base of Cretaceous and the base of Neogene (in the areas where it is entrenched into the HES) are displayed. Geological contacts are taken from Hollard (1985). K: Cretaceous; N: Neogene; HB: Hmam basin; HF: Hmam Fault. QF: Qal’at Fault. For location, compare to Fig. 4.

Geologica Acta, 9(1), 55-65 (2011) 61 DOI: 10.1344/105.000001643 J. GUIMERÀ et al. Topography of a basement massif: the Anti-Atlas

W Ad E

Ô

Ad A N V S

V Ô B

FIGURE 7 Field images of the area studied. pe: Precambrian. Ad: Adoudounian (latest Proterozoic). V: Neogene volcanic rocks. For location, see Fig. 4. A) The Central Anti-Atlas Monocline. In the background, Adoudounian rocks lie horizontal onto the Panafrican basement rocks. In the middleground, slightly folded Adoudounian rocks show a mean dip towards the observer, constituting the monocline limb. The tectonic relief produced by the monocline is 600m. B) North-facing erosional escarpment of the northern slopes of the Jbel Saghro, as seen from the NW. Miocene volcanic rocks lie on top of the escarpment (in the Amalou-n-Mançour mountain) and at its base (in the village of Ikniwn, shown in the photograph). See Figs. 5 and 6 A for location of the escarpment.

Moreover, in the southern limb, this dip is expressed by the western Warzazat basin, so we discard the mantle anomaly structural contours (Fig. 8). as responsible for the difference in elevation between both units. The available data do not allow to ascertain The contouring of the Neogene strata of the Hamada whether the fold is certainly due to a magmatic chamber du Guir evidences a continuous slope descending to the or, alternatively, to a component of W-E (constrictional) S, from 1100m to below 600m (Fig. 8). Consequently, shortening during the Cenozoic orogeny. the Neogene of the Hamada is not affected by the gentle anticline previously described. The steep attitude of thrusts involving Mesozoic or Cenozoic rocks within the Anti-Atlas is due to the fact that they reactivate older faults, probably post-Panafrican INTERPRETATION AND DISCUSSION extensional faults. The history of these faults is long, as they were already reactivated as Variscan thrusts before the The dimensions of the Cenozoic folds of the Anti- Cenozoic deformation (Burkhard et al., 2006; Toto et al., Atlas (hundreds of kilometres in length, and 60-100km 2008; Guimerà and Arboleya, 2008). in wavelength), suggest that they may involve the whole crust or most of it. The Cenozoic folding of the Saghro The Cenozoic N-S horizontal shortening produced anticline and, possibly the Central Anti-Atlas Monocline, associated to the structures represented in cross-sections actually represent the reactivation or tightening of pre- A and B (Fig. 5) is about 1% (E-W line-length shortening existing Variscan anticlines, as originally proposed by across the Sirwa antiform is of even smaller magnitude). Choubert (1952). The origin of the Sirwa N-S anticline This value was obtained by restoring the folds affecting is more debatable. Missenard et al. (2006) interpreted the the different surfaces of reference and comparing them difference in elevation between the basement of the Sirwa to the present horizontal length of their trace in the cross- volcano (where Proterozoic rocks exceed 2300m) and sections. Moreover, the heave (horizontal component of the Warzazat basin floor as the result of the combination the slip, perpendicularly to the fault strike) of the thrusts of a mantle thermal anomaly and a hypothetical mid- was added to obtain the final horizontal shortening. crustal magma injection. However, the wavelength of the lithospheric thinning beneath the Atlas domain (about With regard to the topographic expression of the tectonic 400km according to Teixell et al., 2005 and Fullea et structures, it appears that anticlines give way to elevated al., 2007) exceeds the extent of the Sirwa massif and the areas, in contrast to less elevated regions in intervening

Geologica Acta, 9(1), 55-65 (2011) 62 DOI: 10.1344/105.000001643 J. GUIMERÀ et al. Topography of a basement massif: the Anti-Atlas

synclines or structural depressions. This indicates that caps the Hamada was deposited in a lacustrine environment the local relief of the Anti-Atlas is strongly controlled by (Lavocat, 1954; Thiry and Ben Brahim, 1997) defining a individual deformation structures, likely supported by the paleohorizontal (Babault et al., 2008). This southward slope crustal strength, as suggested by their small amplitude in is also the result of the thermal anomaly, which produced relation to their wavelength. The position of the major the elevation of a wide region extending from the Middle anticlinal hinges determines the location of the fluvial Atlas to the Anti-Atlas (Babault et al., 2008). divides of the Warzazat basin and the Anti-Atlas (Fig. 4), as well as the present-day outlet of the Warzazat basin: the In the Sirwa anticline, two stages of growth can be Draa river occupies a structural low of the Saghro anticline deduced. The first one occurred prior to the deposit of the hinge, as demonstrated by the fact that the HES reaches Aït Kandoula Fm. (Middle to Upper Miocene) and to the 2500m at Jbel Saghro and 2300m at the Jbel Sirwa, while Neogene volcanism. This folding stage created a positive it is at 1400m in the intervening structural depression relief separating the Warzazat and Sous basins. Paleovalleys (Figs. 2, 4 and 5C). were entrenched in this relief, although we cannot tell neither their age nor whether they were remotely connected to the In addition to the local topography, we must recall that sea or formed part of an internally drained system. Later the total elevation of the Anti-Atlas massif is also due to a on these paleovalleys were filled up by the endorheic Aït mantle thermal anomaly put forward by different authors, Kandoula Fm. sediments until the latter overflowed the HES, on the basis of potential-field modelling (notably, of a communicating then the Warzazat basin with the eastern large-scale positive geoid anomaly), the scarce orogenic part of the present-day Sous basin. This is suggested by the thickening, and the widespread alkaline magmatism continuity of Neogene outcrops described earlier (Fig. 6). This (Teixell et al., 2003, 2005; Missenard et al., 2006). A hypothesis necessitates a threshold separating the easternmost component of forebulge uplift in relation to the High Atlas Sous basin from the areas to the west, should we follow thrust loads cannot be discarded, although its signal will be Görler et al.’s (1988) view of the Neogene Warzazat basin as difficult to separate from that of the thermal uplift and of endorheic. The Lower Miocene erosion episode postulated the local tectonic structures. by Missenard et al. (2008) and Balestrieri et al. (2009) in the Marrakech High Atlas after apatite fission tracks could be East of the Saghro anticline, the slope to the south contemporaneous with this first stage of growth. of Cretaceous and Neogene and at the Hamada du Guir (Fig. 8) is not primary. In the case of the Cretaceous During the second folding stage, the continued rocks, the regional slope of the alluvial systems during its growth of the Sirwa anticline deformed the Neogene, as sedimentation was to the North (Guillocheau et al., 2007), deduced from the structural contour pattern (Fig. 6), thus while in the case of the Neogene rocks, the limestone that disconnecting again the present-day Sous basin from the Warzazat basin, leading to the present configuration.

900 As for the Jbel Saghro anticline, constrains on its age N HIGH ATLASA 1100 can be obtained from the erosional escarpment described in A Base�of�the�Cretaceous its North face. This erosional feature implies that the relief A 900 Hamada du Guir Base�of�the�Cretaceous of the Saghro anticline existed during the sedimentation 1100 1000 1200 1000 of the Aït Kandoula Fm., which is accumulated at the 1300 5C escarpment foot. This, together with the presence of late 900 Saghro Anticline No�Cretaceous Miocene (6.6-6.7Ma) volcanic rocks both on top and at the N below�the�Neogene (1100 m) 800 foot of the erosional escarpment indicates that the anticline

600 800 700 and its subsequent erosion formed before the late Miocene, 800 probably in early to mid Miocene times. 900

Base�of�the�Cretaceous 700 The existence of late Cretaceous limestones of similar facies on both sides of the Anti-Atlas and below the Kem-Kem 600 0 100�km Hamada du Guir, dipping away from the Precambrian

Contours�(in�meters)�above�sea-level�of: and Palaeozoic core (Fig. 8), indicates that the Cretaceous 700 Neogene�sedimentary�rocks Base�of�Neogene marine sediments once covered the majority, if not all, of the Proterozoic Base�of�Cretaceous Anti-Atlas region. A Palaeocene-Eocene marine limestone, characteristic of the High Atlas and its forelands, gently FIGURE 8 Map of the region east of the Jbel Saghro with contacts tak- en from Hollard (1985). The contours of the base of the Cretaceous and truncates north-dipping Cretaceous sediments beneath the Neogene sediments are shown. N: Neogene. Location of cross-section Warzazat basin and laps on the Precambrian basement of in Fig. 6 C is shown. For location, see Fig. 1. the basin (Tesón, 2009). This indicates that the Anti-Atlas

Geologica Acta, 9(1), 55-65 (2011) 63 DOI: 10.1344/105.000001643 J. GUIMERÀ et al. Topography of a basement massif: the Anti-Atlas

was already a positive area in the Palaeogene and was not We thank A. Azor and an anonymous referee for comments that covered by these marine sediments. We cannot precise helped to improve the manuscript. when the Saghro anticline developed a critical relief and was eroded into the escarpment, but in the adjacent High , Tesón (2009) described a major hiatus REFERENCES marked by significant deformation and erosion from late Eocene to early-mid Miocene times. Babault, J., Teixell A., Arboleya, M.L., Charroud, M., 2008. A late Cenozoic age for long-wavelength surface uplift of the On the other hand, the existence of Neogene isolated Atlas Mountains of Morocco. Terra Nova, 20, 102-107. buttes at low elevation South of the Saghro massif suggests Balestrieri, M.L., Moratti, G., Bigazzi, G., Algouti, A., 2009. 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Geologica Acta, 9(1), 55-65 (2011) 65 DOI: 10.1344/105.000001643